Modeling Nonlinear Power Amplifiers in OFDM Systems from Subsampled Data: A Comparative Study Using Real Measurements
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Modeling Nonlinear Power Amplifiers in OFDM Systems from Subsampled Data: A Comparative Study Using Real Measurements Ignacio Santamar´ıa Communications Engineering Department (DICOM), University of Cantabria, Avda. Los Castros s/n, 39005, Spain Email: [email protected]
´ Iba´ nez ˜ Jesus Communications Engineering Department (DICOM), University of Cantabria, Avda. Los Castros s/n, 39005, Spain Email: [email protected]
´ Marcelino Lazaro Communications Engineering Department (DICOM), University of Cantabria, Avda. Los Castros s/n, 39005, Spain Email: [email protected]
´ Carlos Pantaleon Communications Engineering Department (DICOM), University of Cantabria, Avda. Los Castros s/n, 39005, Spain Email: [email protected]
Luis Vielva Communications Engineering Department (DICOM), University of Cantabria, Avda. Los Castros s/n, 39005, Spain Email: [email protected] Received 19 April 2002 and in revised form 20 February 2003 A comparative study among several nonlinear high-power amplifier (HPA) models using real measurements is carried out. The analysis is focused on specific models for wideband OFDM signals, which are known to be very sensitive to nonlinear distortion. Moreover, unlike conventional techniques, which typically use a single-tone test signal and power measurements, in this study the models are fitted using subsampled time-domain data. The in-band and out-of-band (spectral regrowth) performances of the following models are evaluated and compared: Saleh’s model, envelope polynomial model (EPM), Volterra model, the multilayer perceptron (MLP) model, and the smoothed piecewise-linear (SPWL) model. The study shows that the SPWL model provides the best in-band characterization of the HPA. On the other hand, the Volterra model provides a good trade-off between model complexity (number of parameters) and performance. Keywords and phrases: nonlinear modeling, high-power amplifiers, OFDM signals, subsampling techniques.
1.
INTRODUCTION
Practical high-power amplifiers (HPAs) exhibit nonlinear behavior, which can become dominant unless the HPA is far from its saturation point. Therefore, to have an accurate nonlinear model for the amplifier is a key factor in order to either evaluate the communication system performance by computer simulation or develop compensation techniques to linearize its behavior (using a predistorter, for instance).
Typically, a power amplifier is represented by nonlinear amplitude (AM/AM) and phase (AM/PM) functions in either polar or quadrature form. These AM/AM and AM/PM curves are measured using a single-tone test signal in the center of the band and they are assumed to be frequency independent (memoryless) over the bandwidth of the communications signal. This assumption limits its use to narrowband applications. A widely used model belonging to this type is Saleh’s model [1], which represents the AM/AM and AM/PM curves by two-parameter formulas. This model can
1220 be extended to wideband signals by considering the model parameters as func
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